Mold, Housing &Wood

Western Wood Products Association

By Coreen Robbins, Ph.D., CIH and Jeff Morrell, Ph.D.

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Authors

Coreen Robbins, Ph.D., CIH is the senior industrialhygienist with Veritox, Inc., a privately owned internationalcorporation based in Redmond, Wash., providing services inindustrial hygiene, human and environmental toxicology andrisk assessment. Dr. Robbins holds a Ph.D. in environmentalhealth science and a master’s degree in occupational safetyand health from The Johns Hopkins University. She is aCertified Industrial Hygienist and is affiliated with the AmericanIndustrial Hygiene Association and the American Conferenceof Governmental Industrial Hygienists.

Jeff Morrell, Ph.D. is a professor in the Department ofWood Science and Engineering at Oregon State University inCorvallis, Ore. He holds a Ph.D. in forest pathology andmycology from the State University of New York College ofEnvironmental Science and Forests, an MS in plant pathologyfrom Pennsylvania State University and a BS in forest biologyfrom Syracuse University. Dr. Morrell has conducted researchon fungi colonizing wood and their effects on properties; bio-logical and chemical control of fungal stain; and control ofwood decay in service.

This paper was prepared by Coreen Robbins, Ph.D., CIHand Jeff Morrell, Ph.D. at the request of Western WoodProducts Association in order to address some common questions about mold and wood. The findings andconclusions contained in the paper represent the work of theauthors, not WWPA. WWPA and the authors assume noresponsibility for any action or inaction based on the contentof this paper, including any liability for damages arising out offailure to remove mold.

TABLE OF CONTENTS

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Why is there concern about mold in homes? . . . . . . . . . . . . 3

What is mold? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Why does mold grow on wood? . . . . . . . . . . . . . . . . . . . . . 4

What types of molds are found on wood? . . . . . . . . . . . . . . 4

How are molds identified? . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

What are “toxic molds”? . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

What is Stachybotrys? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

What are the possible health effects of molds? . . . . . . . . . 6

Where is mold found in buildings? . . . . . . . . . . . . . . . . . . . 7

Do mold spores move from inside walls or floorsinto living spaces? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

How can mold on lumber be prevented? . . . . . . . . . . . . . . 8

When should mold be removed? . . . . . . . . . . . . . . . . . . . . . 8

Can I clean the mold from the wood? . . . . . . . . . . . . . . . . . 8

Once I clean the mold, can it come back?. . . . . . . . . . . . . . 9

Are there mold regulations? . . . . . . . . . . . . . . . . . . . . . . . . . 9

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

© WWPA 2006

Mold, Housing and Wood

Published 2001Revised January 2006

The authors gratefully acknowledge review of the original paper bythe following industry and academic professionals:

Steven E. Carpenter, Ph.D.Abbey Lane Laboratory LLC, Philomath, Ore.

Ed Light, MS, C IHPresident, Building Dynamics LLC, Reston, Va.

Mark L. Nealley, MS, C IHEnvironmental Profiles, Baltimore, Md.

Daniel L. Sudakin, MD, MPHAssistant Professor, Dept. of Environmental and Molecular Toxicology, Oregon State University, Corvallis, Ore.

Photographs, page 5, courtesy of Berlin D. Nelson, Ph.D. and theAmerican Phytopathological Society, from Stachybotrys chartarum:The Toxic Indoor Mold. APSnet, Feb. 2001.

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Introduction

Fungi evolved over 400 million years ago (Sherwood-Pike,1985) and references to mold in buildings suggest that moldshave always been present in human environments. At pre-sent there is growing public concern about the potentialhealth effects of mold in homes and structures that has beenheightened by media reports and litigation.

There are a host of materials in and around the home that,under proper conditions, can become breeding grounds formold. Molds can grow on organic materials such as drywallpaper, wood panels, lumber and carpet backing. They maygrow equally well on inorganic materials such as concrete,glass or plastics that may have nutrients on the surface. In allcases, the presence of moisture is a critical condition for thegrowth of mold.

Lumber is an organic material. In situations with adequatemoisture, mold can become established on wood. However,in all cases involving mold, the underlying problems such asflooding or water leaks affect many materials in the structure,including lumber.

The purpose of this document is to provide information tolumber users regarding the origins of mold growth on wood,types of damage caused by mold and steps that may betaken to prevent, remove and control mold growth.Additionally, this document will also discuss mold-relatedhealth concerns associated with mold exposure.

Why is there concern about mold in homes?

Mold growth in homes has not necessarily increased inrecent years, but new court cases involving mold, sensation-alistic media coverage and publication of questionable scien-tific research have increased public awareness of the issue.

Much of the recent concern about mold was aroused afterseveral articles on the subject appeared in scientific journals.One of the most widely publicized articles was written byresearchers from the U.S. Centers for Disease Control (CDC)(CDC, 1994, 1997). They reported that in 1993, there were10 cases of acute idiopathic pulmonary hemorrhage/hemo-siderosis (AIPH) in infants, some of whom died, that wasthought to be linked to the mold Stachybotrys chartarum(also known as Stachybotrys atra).

This article caused great concern and spurred reactionsacross the country. However, upon closer examination of thestudy and its data and conclusions, a CDC expert panel andan outside expert panel both refuted the initial findings. Bothpanels determined there was no reliable scientific evidencethat Stachybotrys caused the health problems in theseinfants (CDC, 2000a).

While the initial report of the CDC research was widelypublicized, the revised findings received little coverage. As aresult, there continues to be the misperception that there isscientific proof and CDC support for the idea thatStachybotrys chartarum causes serious health problems ininfants. In fact, the CDC noted: “At present there is no test

that proves an association between Stachybotrys chartarum(Stachybotrys atra) and particular health symptoms.” (CDC,2000b)

In the years since the CDC revised the findings of thisstudy, there has been further research and investigation todetermine what adverse human health effects may result frommold exposure in indoor environments. Comprehensivereviews of scientific literature by the Institute of Medicine(IOM, 2004), the American College of Occupational andEnvironmental Medicine (ACOEM, 2002) and the TexasMedical Association (TMA, 2002) have similarly concludedthat an association has not been shown to exist between thepresence of mold or other agents in damp indoor spaces andAIPH.

In its extensive analysis, the Institute of Medicine did notconclude that any adverse health outcomes are caused bythe presence of mold or other agents in damp indoor envi-ronments.

The Institute did find sufficient evidence to conclude thatthere is an association between certain symptoms (upper res-piratory (nasal and throat) tract symptoms, cough, hypersen-sitivity pneumonitis in susceptible persons, wheeze, and asth-ma symptoms in sensitized persons) and mold or dampindoor environments, but the Institute makes it clear that"associated with" does not mean "caused by."

The Institute also found that the evidence is not sufficientto show even an association between the presence of moldor other agents in damp indoor environments and any othersymptom (symptoms examined included shortness of breath,airflow obstruction, mucous membrane irritation syndrome,chronic obstructive pulmonary disease, inhalation fevers, can-cer, skin symptoms, asthma development, gastrointestinaltract problems, fatigue, neuropsychiatric symptoms, lowerrespiratory illness in otherwise healthy adults, and rheumato-logic and other immune disease).

The position from the ACOEM generally agrees with theIOM report in that the health effects from mold in indoor envi-ronments are limited, in most instances, to allergic healtheffects in sensitive persons.

Two other articles have influenced the concern about moldand health problems, specifically in persons who worked inoffice buildings. These studies claimed to show a causal linkbetween working in the buildings and symptoms such asheadache, fatigue and cough, as reported in questionnaires(Johanning, 1996; Hodgson, 1998). The authors of thesestudies concluded that mycotoxins were the root of thehealth symptoms. However, when these reports were exam-ined closely, they were found to have many limitations andthe data did not support these conclusions (Fung, 1998;Page, 1998; Robbins, 2000).

In addition to the above studies, there are some widelycited anecdotal reports of acute, or sudden, health effectsattributed to mycotoxins (toxins produced by molds) afterexposure to extremely moldy conditions (Brinton, 1987; Croft,

Wood also contains a variety of other materials, includingsugars, starches, proteins, lipids and fatty acids. These mate-rials are present in the storage tissues of the living tree andare essential for a variety of functions. Even after a tree is har-vested, these materials remain in the wood and can providethe initial food source for mold fungi.

Mold fungi are rarely present inside a living tree becausethe bark provides an excellent barrier against fungal andinsect attack. Once the tree is harvested, these protectiveeffects decline and the many spores present in the air cansettle on the surface and colonize the wood. Also, the foodsources for mold—the stored sugars, starches and othercompounds—are exposed when logs are processed intolumber.

What types of molds are found on wood?

Under the proper conditions, wood may be colonized by avariety of fungi (Davidson, 1935; Dowding, 1970; Kaarik,1980). A recent study at Oregon State University revealedthat Douglas fir sapwood was colonized by over 45 speciesof fungi within six weeks after sawing (Kang, 2000). Most ofthese fungi are common to many other materials, while a fewwere specialized and only grow on wood.

Molds and stain fungi are the most rapid colonizers offreshly exposed wood. Both fungi discolor the wood and arealmost indistinguishable from each other to the naked eye.

Molds are typically characterized as fungi that discolor thewood surface through production of pigmented spores thatcan be yellow, green, orange, black and an array of other col-ors. The discoloration seen with molds is usually confined tothe wood surface.

Stain fungi discolor the wood more deeply and are not aseasily removed. These fungi may produce some discolorationas they grow on the wood surface, but the primary changesoccur as they grow deeper into the wood. Stain fungi darkenas they age. This darkening creates what is called “blue stain”in the wood (Zink, 1988). Stained wood can experience minorlosses in physical properties, but the primary changes are incolor and the increased ability to absorb liquids (Lindgren,1952).

Decay fungi may also grow when wood products areexposed to chronic moisture. Decay fungi attack beyond thesurface of the wood into the structural polymers of the fiber,reducing its strength. Decayed wood may be discolored, butspores of the decay fungus are not typically found on the sur-face. Spores of most species are produced on more complexfruiting structures that can produce billions of spores.Generally, decay fungi invade wood in structures after pro-longed exposure to moisture, such as what occurs withplumbing leaks or seeping from outdoor water sources.

Many of the molds and other fungi that grow on wood arefound on almost any material containing sugars or starches,including plant leaves, bread and other foods. They can grow

1986; Di Paolo, 1994; Emanuel, 1975; Malmberg, 1993).There is little information in these reports to relate the healtheffects to mycotoxins and no measurement of exposure. In allof them, the individuals were likely exposed to high concen-trations of mold spores but recovered after they wereremoved from exposure.

Other symptoms, such as nervous disorders, memory lossand joint pain, are attributed by some to molds. However,there is no credible evidence in the medical and scientific lit-erature that supports the link between molds and thesehealth problems (ACOEM, 2002; IOM, 2004; Page, 2001;Robbins, 2000).

What is mold?

Molds are part of the fungi kingdom. Fungi are a diversegroup of organisms within a wide range of species thatinclude mushrooms, bracket fungi, molds and mildew.Distinguishing features of fungi are the need to extract theirfood from the organic materials they grow on and the abilityto reproduce by way of minute spores. Fungi are a part ofnature's recycling system and play an important role in break-ing down materials such as plants, leaves, wood and othernatural matter.

Mold is the common name for many types of micro fungi.In order to grow, molds require food, suitable temperature(ideally between 70 and 85 degrees Fahrenheit), oxygen andmoisture (Zabel, 1992). When these conditions are met, moldwill grow and reproduce by creating spores that are releasedinto the air. Molds are very adaptable and can grow even ondamp inorganic materials such as glass, metal, concrete orpainted surfaces if a microscopic layer of organic nutrients isavailable. Such nutrients can be found in household dust andsoil particles.

Conservatively, more than 100,000 species of mold existin the world. At least 1,000 mold species are common in theU.S. (Hawksworth, 1999). It is estimated that molds andother fungi make up some 25 percent of the earth's biomass.Most mold spores land on places unsuitable for growth andeventually die. A select few land on surfaces containing nutri-ents and where the moisture, oxygen and temperature condi-tions are right for growth.

Mold and mold spores are everywhere around us andhave always been a part of our environment. The air webreathe is a virtual jungle of fungal spores. We routinelyencounter mold spores as part of everyday life both indoorsand outdoors. Spore levels may vary seasonally, but somespores are always present.

Why does mold grow on wood?

Wood is a biological material consisting primarily of cellu-lose, lignin and hemicellulose. These three structural polymersmake up 90 to 99 percent of the wood mass and give woodits unique properties that make it an excellent structuralmaterial (Panshin, 1980).

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on a microscopically thin layer of organic material, even form-ing on common household dust.

These fungi have evolved to rapidly colonize a substrateand utilize the stored sugars as quickly as possible, but theylack the ability to cause significant effects on the wood struc-ture. The most common effect of mold attack on wood is anincrease in permeability, which can lead to an increase inmoisture or paint uptake (Lindgren, 1952).

How are molds identified?

Mycology is the scientific study of fungi. Proper identifica-tion of molds requires that the person examining the fungihave extensive professional training in mycology. Althoughsome species produce distinctive structures or colors, it isnearly impossible to identify the fungi present on wood withthe naked eye. The identification of fungi from a sample usinga microscope can take a few days or several weeks, depend-ing on the species.

Most mold and stain fungi are identified by the spores theyproduce and the structures on which they are produced.Samples can be taken by smoothing a piece of clear tape onthe wood surface, then mounting the tape on a microscopeslide. Another approach is to cut small pieces from the woodsurface, then place these on a nutrient media. Fungi growingfrom the wood onto the media are then examined under amicroscope for spores and other key identifying features.

It is important to note that finding mold does not provideinformation about the possible exposure to mold, or the riskof health effects from mold. The airborne mold spore concen-tration, or possible exposure to mold, cannot be calculatedfrom the types and quantity of molds found on surfaces.

In addition, many homeowners ask to have molds identi-fied to species. In most cases, this is unnecessary and cost-ly. Molds are a moisture indicator and should be dealt with assuch. Eliminating the moisture source and cleaning theaffected surfaces generally negates the need for identification.So-called “mold test kits” should be used with caution andthe results interpreted carefully since sampling accuracy is animportant aspect in using these kits.

A visual inspection is usually the most effective method fordistinguishing clean and moldy environments. In the absenceof visible mold growth, sometimes the air is sampled to esti-mate the number of airborne mold spores.

Air sampling can be expensive and results are difficult tointerpret in terms of what is a “normal” environment and whatis the potential for health effects. Air sample results only tellwhat the airborne levels are at the sampling time, providingonly a “snapshot in time” of airborne spores. Results arehighly variable, due to the natural variability of the environ-ment and the sampling and analysis methods (Baxter et al.,2005).

In general, normal indoor environments are expected tohave mold spore levels similar to or less than outdoors. This

is because the outdoor air normally is the dominant source ofspores in the indoor air.

The most important limitation of air sampling is that thereare no health-based standards for mold exposure levels inindoor air, so there is nothing with which to compare the airsample results; and therefore, no way to determine thepotential risk of effects from the amount of airborne moldspores found (Terr, 2004).

Individuals who inspect and test homes for mold shouldhave the appropriate education and experience. A certifiedindustrial hygienist (CIH) with experience in sampling formolds is generally qualified to inspect a home for the pres-ence of visible mold, to collect samples for mold if necessaryand to help with interpretation of the results. Industrial hygien-ists have training in exposure assessment and methods ofcontrolling exposures to molds and other dusts. They canalso provide advice on how to control exposure and contami-nation during clean up of mold.

The American Board of Industrial Hygiene sets standardsfor certifying hygienists. These standards require at least abachelor's degree with a minimum 30 semester hours of sci-ence and specific industrial hygiene coursework, a minimumof four years of professional level-1 industrial hygiene experi-ence and successful completion of a comprehensive one-dayexamination.

What are “toxic molds”?

Some molds are referred to as "toxic molds" becauseunder certain conditions, they can produce mycotoxins.Mycotoxins are compounds "produced by fungi that are toxicto humans or animals and have economic impact." (Ciegler,1980). Many common molds can produce mycotoxins. Thosethat arbitrarily have been cited as "toxic molds" includeStachybotrys chartarum (or atra), and various species ofAspergillus, Fusarium and Penicillium.

Mycotoxins are secondary metabolites. This means thatthe mold does not need to produce mycotoxins to grow orsurvive. Mycotoxins are produced only when certain environ-mental conditions are in place and when produced are foundin extremely small quantities on a per-spore basis.Mycotoxins are contained in the spore itself and also may befound in the substrate or material in which the mold is grow-ing (Jarvis, 1986).

The isolation of a mold type that has shown to producetoxins (“toxigenic” species) does not substantiate thepresence of mycotoxins (Ren, 1999). For example, knownmycotoxin-producing strains of Aspergillus flavus and A. fumi-gatus were grown on various building and construction mate-rials. No mycotoxins were found in extracts of densely colo-nized ceiling tiles, wallboard, wallpaper and air filters. Thesenegative results were obtained even with enhanced growthwhen the indoor construction and finishing materials weresupplemented with carbon and nitrogen (Tuomi, 2000).

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or confirmed by objective scientific data. In these anecdotalstories, symptoms are also reported to cease after the indi-vidual is no longer exposed.

What are the possible health effects of mold?

Humans are exposed constantly to molds in the environ-ment, whether indoors or outdoors. Our immune systemsand respiratory clearance systems normally provide defensemechanisms that protect us from health effects of airbornemolds. Problems arise when the immune system is sup-pressed (HIV infection, cancer treatment), over-responsive(allergy) or when exposures are exceedingly high (irritationand mycotoxin effects).

Mold infections are possible in people with immune sys-tem supression, but this has not been reported to occur dueto mold in residential settings. Some people with compro-mised immune function, for example people undergoingchemotherapy or organ transplants or people with AIDS, maybe at increased risk for opportunistic infections from commonmolds both indoors and outdoors.

Many people are allergic to molds and allergic responsesinclude hay fever and asthma. About 5 percent of theAmerican population experience allergic symptoms due tomolds and other fungal antigens (fungi other than molds pro-duce spores that may provoke allergic responses) (ACOEM2002). The most common allergies are to the abundant out-door molds, such as Cladosporium, Aspergillus andPenicillium. Allergic responses to molds tend to be allergicasthma or hay fever.

Molds can produce volatile organic compounds (VOCs)that can be irritating when present in high concentrations.However, the concentration of the VOCs is usually too low,even in damp and moldy buildings, to cause sensory irritationsymptoms such as burning eyes and upper airway irritation(Pasanen, 1998; Korpi, 1999). On the other hand, theseVOCs have very low odor thresholds and those odors may benoticed and annoying at levels well below the concentrationsthat might result in irritation (Pasanen, 1996; 1998).

Routes of exposures to mold include dermal (skin con-tact), ingestion and inhalation. The route of exposure has aprofound effect on the dose, or amount of material or toxinabsorbed by the body.

Dermal exposure to mold occurs when the skin is in con-tact with mold colonies or mold spores. The spores do notpass through the skin, but may cause irritation if there is con-tact with large amounts of spores or moldy material(Dobrotko, 1945). The irritation may be from reaction to aller-genic compounds or chemicals, including mycotoxins, andpossibly from rubbing against the spores themselves. Skin isgenerally a good barrier against particles. Since mycotoxinsstay with the particles, the skin is not a significant route ofexposure for mycotoxins.

Mold spores or particles that become airborne can beinhaled. Bigger particles are stopped in the upper airways of

Mycotoxins are relatively large and heavy molecules(Schiefer, 1990; WHO, 1990). This means they are not volatileand do not evaporate from the mold spore or substrate parti-cle. The musty odor associated with mold comes fromvolatile compounds generated as the mold reproduces(Pasanen, 1996). These compounds, which are different fromthe mycotoxins, may be annoying and irritating, but are notmycotoxins and are not highly toxic. The concentration of thevolatile organic compounds (VOCs) is usually too low, even indamp and moldy buildings, to cause sensory irritation symp-toms such as burning eyes and upper airway irritation(Pasanen, 1998; Korpi, 1999). However, the odor of thesecompounds may be noticeable at levels well below the con-centrations that might result in sensory irritation (Pasanen,1996; 1998).

What is Stachybotrys?

Stachybotrys is a mold that grows well on chronically wetcellulose material. Outdoors, Stachybotrys typically grows onand breaks down dead plant material. In research conductedby Oregon State University, none of the 45 different speciesof fungi growing on samples of Douglas fir sapwood wereStachybotrys chartarum (Kang,2000). Stachybotrys chartarumand Stachybotrys atra are twodifferent scientific names for thesame mold.

Indoors, it can grow on chron-ically wet cellulose building mate-rials such as lumber, wood pan-els, drywall backing, insulationand ceiling tiles. Moisture con-tributing to indoor growth ofStachybotrys is usually providedby flooding or leaks. Left wet, thespores of Stachybotrys are noteasily released to the air.However if allowed to dry, thespores may become airborneand can be inhaled.

The refuted CDC article andother questionable scientificpapers directed much attentionto Stachybotrys. However, thesestudies provide no credible sci-entific evidence of an associationbetween chronic, low-levelStachybotrys exposure andhealth effects (Page, 2001). Thefew reports that associate directcontact and exposures to highconcentrations of Stachybotryswith symptoms of skin rash, coughing, chest tightness,bloody nose, fatigue, nausea and—in rare cases—lowerwhite blood cell counts are anecdotal, and are not supported

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Fig. 2: The mass of conidiaof S. chartarum at the tip ofthe conidiophore. Scanningelectron micrograph.Courtesy of Berlin D.Nelson, Ph.D. and APSnet

Fig. 1: A conidium of S.chartarum showing theridged surface of a maturespore. Scanning electronmicrograph.Courtesy of Berlin D.Nelson, Ph.D. and APSnet

the nose or mouth, and in the trachea and bronchi. Generally,only the smallest particles—those smaller than 5 microns—are able to reach the lungs.

Although mycotoxins can be inhaled, mold spores aresmall and the amount of toxin in each is tiny. For most peo-ple, the airborne concentration needed to get to a toxic doseis in the range of many hundreds of millions of spores percubic meter of air (Kelman, 2004).

Ingestion is a more direct exposure route for mold.Historical incidents of mass human poisonings from moldshave always involved the eating of moldy foods (Hudler,1998). Through ingestion, a much larger mass of mold—andany mycotoxins present—can be taken into the body ascompared to the inhalation route.

Framing lumber in a newly finished house is typicallyencased by panels or siding on the outside and drywall orpanels on the inside. As such, there is virtually no chance foroccupants in a home to be exposed to any mold on thewood through skin contact or ingestion.

Inhalation exposure to mold on framing lumber in a fin-ished home is possible, but not very likely. Mycotoxins are notvolatile, so they cannot "off-gas" into the environment ormigrate through walls or floors independent of a particle.Since particles cannot move through solid objects, mycotox-ins in molds contained inside a wall or floor cavity will staythere unless disturbed.

Where is mold found in buildings?

The presence of molds in our everyday environmentmeans they can grow anyplace under the proper conditions.In all cases, moisture is the essential element for mold growthin buildings.

There are many potential sources for unwanted moisturein buildings. For example, improperly maintained air condi-tioning systems that create excessive condensation can be abreeding ground and distribution mechanism for moldparticles.

Mold growth may be found in walls, ceilings and floor cavi-ties when standing water is produced in a building or gets inand stays for more than a few days. Sources for water tosupport molds and other fungi in homes include plumbingleaks, gaps in roofs, siding or masonry, poorly sealed win-

dows, porous slabs and foundations, inadequate drainage,and faulty roof drains and downspouts.

Poor ventilation and/or air circulation combined with highindoor humidity—from showers, cooking or other activities —can result in condensation that promotes mold growth oncooler surfaces. Poorly insulated walls may also provide asurface for condensation and mold growth in buildings thatdo not have general humidity problems.

Surface moisture on unseasoned framing lumber, appear-ing as the wood dries, may create conditions for moldgrowth. However, once the moisture content of the wood fallsbelow 20 percent, mold growth can no longer be supported.Depending on the climate, framing lumber will dry to below20 percent moisture content during the construction andbefore the building is enclosed.

In instances where wood is chronically exposed to water,wood decay fungi can invade. Decay fungi can penetratemore deeply and attack the structural polymers in the fiber,reducing the strength of the wood.

Other conditions can increase the amount of mold sporesin the indoor air of buildings. Homes with exposed-dirt crawlspaces and basements tend to have more airborne moldspores than homes without (Lumpkins, 1973; Su, 1992). Withthe right humidity conditions, some molds can grow onhouse dust. It is not surprising, then, that poor housekeepingand high indoor humidity are both associated with increasedlevels of airborne mold spores (Solomon, 1975; Kozak,1979).

The biggest source of indoor mold spores is often the out-door air (Solomon, 1975). Higher levels of indoor mold sporestend to be found in homes with yards having dense and over-grown landscaping (Kozak, 1979).

Indoor mold levels are generally lower in buildings withforced-air heating systems (as opposed to window ventilation)and lower still when these systems include a well-maintainedand properly functioning air conditioning system.

Mold spore levels outdoors vary with the season andweather. They may be very high in the growing season orapproach zero when snow covers the ground (Solomon,1975). Except for the snow-cover situation, mold spores innormal indoor environments are usually between 20-50 per-cent of the outdoor levels.

Do mold spores move from inside walls orfloors into living spaces?

Mold spores may be present in a home but out of sightinside a wall or floor cavity. This mold may have grown duringconstruction and, with the wood dry and moisture gone, bedead or dormant. It may also grow after a leak has beenrepaired.

There is limited research about whether such "hidden"mold can move into the structure and increase the amount ofmold spores in the living space. The authors of a recent

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Fig. 3: Mold on lumber

study, which included analysis of data from 200 homes, con-cluded that the presence of Stachybotrys inside intact walls isnot correlated with Stachybotrys found in the living spaces.

The Wisconsin Department of Health and Family Servicesinvestigated the relationship between mold on surfaces of ori-ented strand board siding and mold levels inside the home.The results of the study indicated that mold levels in theaffected homes were not significantly higher than those mea-sured in non-exposed homes (Dagger, 1999).

In Veritox’s experience sampling many buildings withchronic water leaks and large amounts of enclosed mold(usually more than 10 percent of internal wall area containsmold or wood rot), mold concentrations found indoors aresimilar to or less than the levels found outdoors. From this itis reasonable to infer that small amounts of mold enclosed inwalls, floors or ceilings will not have a large impact on theindoor air quality.

How can mold on lumber be prevented?

All fungi have four basic requirements for growth: suitabletemperature, oxygen, food and moisture. Eliminating one ofthese required elements can prevent fungal growth (Scheffer,1940; 1973).

Mold fungi have fairly broad temperature requirements butmost grow best at temperatures between 70 and 85 degreesFahrenheit. Most fungi require oxygen to function. In fact, onemethod for preventing stain and mold in wood is to sub-merge it in fresh water, which fills the wood cells with waterand limits the availability of oxygen. Lumber and wood prod-uct mills often utilize this method by spraying log decks withwater or storing logs in ponds at the plant.

While controlling temperature or oxygen is generally notpractical for wood products, it is possible to remove moistureas quickly as possible during manufacturing. Reducing themoisture content of lumber to less than 20 percent will signifi-cantly decrease the opportunities for mold to form on thewood.

Drying lumber reduces the likelihood of mold formation.But it does not guarantee the wood will remain free of mold.Lumber that is exposed to moisture after it has been driedwill support mold growth.

Dry lumber can become wet through direct sources, suchas rainfall or condensation. Even dry lumber contains somemoisture. So, wet pieces inside wrapped bundles of lumbercould create conditions for mold growth. Exposing the bundleto direct sunlight, for example, could heat the lumber and thewrapping may trap the evaporating moisture. This trappedmoisture can be sufficient to support mold growth.

Each year, billions of board feet of lumber are sold asunseasoned, or green products and are allowed to dry natu-rally, usually during the framing stages of building a house.Many mills reduce the risk of mold and stain on green lumberby applying antistain, or sap stain treatments, which are thin

coatings of fungicides on the wood surface. These fungicidesare applied by dipping entire bundles of lumber into a treat-ment solution or by spraying all four surfaces of individualboards (Scheffer, 1940).

These chemicals are designed to provide a microscopicbarrier against fungal attack that lasts for three to six months,depending on the chemical, the concentration used, thewood species and the climatic conditions. The chemicalsused for preventing mold and stain are usually very mild andinclude many used on food crops as well as in shampoosand paints. They are not designed for long-term protection ofthe wood.

When should mold be removed?

Visible mold growing on surfaces where people may comein contact with it should be cleaned and removed. The deci-sion to remove mold from enclosed cavities must be madeafter considering how much mold is likely to be present andhow likely it is to be opened or disturbed. In some cases,wood can simply be treated for mold growth with a bleachsolution, then dried and sealed.

Where mold is present in existing structures, there areoften reasons for opening walls and removing building partsthat are unrelated to mold growth (such as for repairingwarped and water-damaged floors or walls). In buildingswhere mold removal from enclosed cavities is not desirable orfeasible, sampling can be conducted to monitor the level ofmold spores in the occupied spaces.

The process of removing mold from enclosed spacescould increase exposure to spores in the short term. Highindoor mold spore levels are sometimes found when wallsand floors containing mold are opened or disturbed, andwhen visible mold growth is present on exposed surfaces.

Can I clean the mold from the wood?

The decision to clean mold from lumber depends on theamount of mold present and how likely it is to be disturbed.In nearly all cases, mold cleaning should be undertaken onlyafter any moisture problems are resolved.

For any mold clean up that may generate large amountsof dust, basic personal protection equipment such as rubbergloves, eye protection and a high-quality pollen or dust maskshould be worn. Clean-up of small spots or areas of moldgenerally does not require any special protective equipment.

There are a number of products on the market, from com-mercial mildewcides to common bleach, which are promotedfor removing mold from wood. However, the U.S.Environmental Protection Agency suggests using mild deter-gent and water for most mold clean up. For cleaning woodsurfaces, the EPA recommends wet vacuuming the area,wiping or scrubbing the mold with detergent and water and,after drying, vacuuming with a high-efficiency particulate air(HEPA) vacuum (EPA, 2001).

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The molds seen on lumber are largely a collection of fun-gal spores on the surface of the wood. As such, wet wipingor scrubbing the lumber will remove the mold.

Simply wiping the wood, however, can release thosespores into the surrounding air. A better approach is to gentlyspray or wet down the mold prior to removal. Once the moldhas been wetted, it can be removed by wet-wiping the sur-faces with a water and detergent solution, scrubbing if nec-essary.

If commercial products are used for cleaning mold, besure to follow the manufacturer's instructions for use.Common bleach also can be used, particularly to clean thediscoloration caused by mold fungi. The U.S. Centers forDisease Control recommends using a solution of 10 partswater to one part chlorine bleach to clean mold from surfaces(CDC, 2000b). When using bleach and other cleaning chemi-cals indoors, make sure there is adequate ventilation andwear personal protection equipment outlined previously.Never mix bleach with ammonia.

Removing small amounts of mold from wood is relativelystraightforward. Mold removal becomes more complex whenthere are heavy amounts of growth on a majority of the lum-ber or if the building has been in service for some time andthe mold originated from leaks into the building cavity. Inthese instances, the mold clean up should be done by a pro-fessional cleaning and restoration company.

Once I clean the mold, can it come back?

Mold spores are present on surfaces in all homes, socleaning will not prevent re-growth of mold (Taylor, 2004).Even if a building is stripped of all components and everyspore is killed or removed, normal background mold sporesfrom outdoors or on replacement parts have the potential togrow.

The most important objective in any mold removal is toeliminate any sources of moisture. Should the wood framingin a house become wet, through leaks or flooding, it is imper-ative that the area be dried as soon as possible.

In many climates, this drying will occur naturally once anystanding water is removed. In other climates where the rela-tive humidity is higher, it may be necessary to bring inportable fans to increase airflow or to use the existing heatingsystem or portable electric heaters to encourage faster dry-ing.

Are there mold regulations?

In the U.S., there are currently no regulations or exposurelimits for molds or mycotoxins. This is true for homes, occu-pational settings, schools, stores and other public buildings.

In the occupational setting, the general duty clause mayapply to mold exposures. This is the rule that requiresemployers to provide workers with a safe and healthy workenvironment. The Occupational Safety and Health

Administration (OSHA) should be consulted for specific infor-mation on work-related mold questions.

Because there are no exposure limits for molds, there areno "benchmarks" with which to compare exposure measure-ments. Typically, measured indoor airborne mold levels arecompared to outdoor concentrations. Differences betweenthe types and numbers of molds indoors vs. outdoors canprovide clues as to whether the exposures indoors are abovethe background level and whether there is a source of moldinside the building. However, these data usually cannot beused to determine if exposure levels are safe. In most cases,air sampling for mold is not needed to assess or remediate amold problem.

Summary

Molds play an important role in nature by breaking downorganic materials. We routinely encounter mold spores aspart of everyday life, in both outdoor and indoor environ-ments. In most cases, the body's immune and respiratorysystems normally provide defense mechanisms that protect itfrom health effects of regular exposure to molds.

Inhalation of molds can result in a range of health effectsin some circumstances. Infections are possible in immune-compromised people, although there are no reports of thisoccurring from mold growth in a residence. Allergic respons-es to molds include hay fever and asthma, and many peoplewith allergies are also allergic to mold. The amount of moldthat must be inhaled to cause an allergic response isunknown. Toxic effects from inhalation of mold may occur insituations where there is prolonged exposures to exceedinglyhigh airborne mold concentrations, such as in an agriculturalsetting. These high concentrations have not been reported tooccur in residences with mold enclosed in finished walls.

Lumber is just one of thousands of materials that can be apotential growth substrate for mold under the properconditions. In a vast majority of cases, mold problems inhomes are related to flooding or water leaks that affect manymaterials in the structure, including lumber.

Moisture is essential for mold growth and controlling mois-ture offers the best protection against mold. While all woodcontains moisture, mold growth is not supported on wooddried to below 20 percent moisture content. Lumber used inconstruction will typically dry to below 20 percent moisturecontent before the structure is enclosed.

Drying lumber does not guarantee the wood will remainfree of mold. If lumber is exposed to moisture after it hasdried, it can provide a surface for mold to grow.

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Western Wood Products Association522 SW Fifth Avenue, Suite 500Portland, Oregon 97204-2122 USA

tel: 503-224-3930fax: 503-224-3934e-mail: info@wwpa.orgweb site: www.wwpa.org

TG-2/0807/Rev.2-06/2000